Twisted magnetization states near the compensation temperature of $\mathrm{Fe}∕\mathrm{Gd}$ multilayers: Anisotropy and surface-termination effects

Inhomogeneous magnetization depth profiles in antiferromagnetically coupled $\mathrm{Fe}∕\mathrm{Gd}$ multilayers are investigated using x-ray magnetic circular dichroism (XMCD) measurements. Spin configurations are studied as a function of temperature and applied field on two multilayers with different termination layers. We place particular emphasis on the vicinity of the ferrimagnetic compensation temperature $\left(T_{\mathrm{Comp}}\right)$, where the Gd and Fe magnetizations are equal but opposite. While away from $T_{\mathrm{Comp}}$ the spin configurations are dictated by the presence of surface- and bulk-twisted states, unusual field dependence is observed near $T_{\mathrm{Comp}}$. We used micromagnetic simulations to show that the unusual observed phenomena near $T_{\mathrm{Comp}}$ are due to a small uniaxial anisotropy in the Fe layers, whose influence on the spin configurations becomes apparent only near $T_{\mathrm{Comp}}$. The simulations also show the onset of the surface- and bulk-twisted phases at temperatures below and above $T_{\mathrm{Comp}}$ in agreement with the experimental XMCD data.

Figure 1

SQUID magnetization measurements on the Fe-terminated and Gd-terminated $[\mathrm{Fe}∕\mathrm{Gd}]$ multilayers. The ferrimagnetic compensation temperatures are approximately 85 and $95\mathrm{K}$ for the Fe- and Gd-terminated samples, respectively. The applied field is $500\mathrm{Oe}$.

Figure 2

Electric field intensity profiles inside the multilayers at a grazing incident angle of 0.355°. Positive values for the depth from the surface correspond to inside the multilayers, and negative values correspond to air. The first $50\mathrm{\AA }$ corresponds to an Al capping layer. The incoming x-ray energies are 7.112 $\left(\mathrm{Fe}K\right)$ and 7.247 $\left(\mathrm{Gd}{L}_3\right)$ keV. Only the top half of the multilayer is shown for each case. (a) Gd-terminated sample and (b) Fe-terminated sample.

Figure 3

Element-specific XMCD hysteresis loops from the Fe-terminated sample at the indicated temperatures. The XMCD signal is recorded as an asymmetry ratio as defined in Sec. 2B of the text. The first column in each plot shows hysteresis loops for the top layers, and the second column is for the total layers. For clarity, the hysteresis loops are shifted down vertically by multiples of 0.04. The loops at $80\mathrm{K}$ are shaded to indicate the temperature is near the compensation temperature. The arrows in (a) and (b) indicate the path of the curves. (a) Fe hysteresis loops, (b) Gd hysteresis loops, and (c) experimental geometry. The XMCD signal $\propto \mathbf{k}\bullet \mathbf{M}$, where $\mathbf{k}$ is the incident x-ray wave vector and $\mathbf{M}$ is the sample magnetization (Gd-XMCD $\propto \mathbf{k}\bullet {\mathbf{M}}_{\mathrm{Gd}}$ and Fe-XMCD $\propto \mathbf{k}\bullet {\mathbf{M}}_{\mathrm{Fe}}$). The applied field is $\mathbf{H}$, $\theta$ is the x-ray incident angle, and $\varphi$ is the in-plane canting angle of the layer magnetization.

Figure 4

Element-specific hysteresis loops from the micromagnetic simulations on the Fe-terminated multilayer. The loops are shifted down vertically by multiples of 3 for clarity. The loops at $82\mathrm{K}$ are shaded to indicate the temperature is near the compensation temperature. For $T=70$, 82, and $99\mathrm{K}$, simulation results without the Fe anisotropy are shown as dashed lines for comparison. (a) Fe hysteresis loops. (b) Gd hysteresis loops.

Figure 5

Simulated spin configurations for the Fe-terminated sample slightly above $T_{\mathrm{Comp}}$ $(T=82\mathrm{K})$ at the selected field values as field is swept as in Fig. 4. For each sublayer, the magnetization angle $\varphi$ with respect to the applied field direction ($H\Vert 0°$ or $H\Vert 180°$) is plotted as a function of depth from the surface. The circles represent the Gd moments, and the crosses are for the Fe moments. The uniaxial anisotropy of Fe layers is along $\varphi =30°$. (a) At $-400\mathrm{Oe}$, (b) at $-160\mathrm{Oe}$, (c) at remanence, (d) at $+80\mathrm{Oe}$, (e) at $+160\mathrm{Oe}$, and (f) at $+400\mathrm{Oe}$.

Figure 6

Simulated spin configurations are shown for the Fe-terminated sample below $T_{\mathrm{Comp}}$ $(T=41\mathrm{K})$ at the selected field values as the field is swept as in Fig. 4. For each sublayer, the magnetization angle $\varphi$ with respect to the applied field direction ($H\Vert 0°$ or $H\Vert 180°$) is plotted as a function of depth from the surface. The circles represent the Gd moments, and the crosses are for the Fe moments. The uniaxial anisotropy of Fe layers is along $\varphi =30°$. (a) At $-400\mathrm{Oe}$, (b) at $-160\mathrm{Oe}$, (c) at remanence, (d) at $+80\mathrm{Oe}$, (e) at $+160\mathrm{Oe}$, and (f) at $+400\mathrm{Oe}$.

Figure 7

Spin configurations from simulations for the Fe-terminated sample above $T_{\mathrm{Comp}}$ $(T=99\mathrm{K})$ at the selected field values as the field is swept as in Fig. 4. For each sublayer, the magnetization angle $\varphi$ with respect to the applied field direction ($H\Vert 0°$ or $H\Vert 180°$) is plotted as a function of depth from the surface. The circles represent the Gd moments, and the crosses are for the Fe moments. The uniaxial anisotropy of Fe layers is along $\varphi =30°$. (a) At $-400\mathrm{Oe}$, (b) at $-160\mathrm{Oe}$, (c) at remanence, (d) at $+80\mathrm{Oe}$, (e) at $+160\mathrm{Oe}$, and (f) at $+400\mathrm{Oe}$.

Figure 8

(a) For the Fe-terminated sample, VSM hysteresis loops are measured at different sample orientations, and coercive fields $H_C$ as a function of the sample orientation are plotted. (b) The anisotropy direction from VSM along with the XMCD measurement direction are shown with respect to the Fe-terminated film.

Figure 9

XMCD hysteresis loops from the Gd-terminated sample at the indicated temperatures. The XMCD signal is recorded as an asymmetry ratio as defined in Sec. 2B of the text. The first column in each plot is for the top layers, and the second column is for the total layers. The plots are shifted vertically for clarity. (a) Fe hysteresis loops. (b) Gd hysteresis loops.

Figure 10

Micromagnetic simulation on the Gd-terminated multilayer. The plots are shifted vertically for clarity. (a) Fe hysteresis loops. (b) Gd hysteresis loops.